In Patent Document 1, a technology to detect a state of charge (SOC) of a lead-acid battery is disclosed. In this technology, a relational expression between a terminal voltage and a discharge current is found in advance as for a lead-acid battery whose SOC is already known, and a coefficient of the relational expression is found from a state when the SOC is different. The terminal voltage and the discharge current are applied to the found relational expression, and thereby, it is possible to find the SOC of the lead-acid battery at an arbitrary time.